1. Field of the Invention
The present invention relates to a steplessly variable vehicle transmission and more particularly to a speed ratio control of such a vehicle transmission. More specifically, the present invention pertains to an automatic speed ratio control system for a steplessly variable vehicle transmission.
2. Description of the Prior Art
Conventionally, it has been known to control a steplessly variable vehicle transmission automatically in accordance with the engine speed and the engine load so that the speed ratio of the transmission be adjusted to a value wherein the most efficient engine operation can be performed to thereby improve fuel economy. For example, Japanese patent publication 45-32567 discloses a transmission provided with a speed ratio control system including a control value which is associated with a governor mechanism responsive to the engine speed and a spring of which strength is adjusted in accordance with the engine load, such as the engine throttle valve opening.
In vehicles provided with this type of automatic transmissions, there is a problem that the response of the vehicle to an operation of the engine power control member such as the throttle valve control foot pedal is noticeably different from that of vehicles provided with manually operated gear transmissions. With such gear transmissions, when the engine control member is actuated to accelerate the vehicle, the engine speed is increased proportionally to an increase in the vehicle speed so that the driven can obtain a feeling of desired acceleration. In the vehicles having the aforementioned steplessly variable transmissions, however, the speed ratio of the transmission is automatically adjusted so that the engine is operated with a speed which is determined in relation to the position of the engine control member, for example, the engine throttle valve opening. Therefore, it is impossible to obtain a feeling of rapid acceleration as in the vehicles having gear transmissions.